Chlorine is electrolytically prepared from sodium chloride in electrolytic cells of the diaphragm type. In diaphragm chlor-alkali electrolytic cells, brine is fed to the cell chamber. Within the anolyte chamber of the cell, an anolyte cell liquor containing sodium ion and chloride ions is present. The chloride ions react at the anode according to the following reaction: EQU Cl.sup.-.fwdarw.Cl + e.sup.-
to provide nascent chlorine which then reacts: EQU 2Cl.fwdarw. Cl.sub.2
to provide molecular chlorine.
The sodium ion passes through a permeable barrier to the catholyte chamber. In one type of cell, the permeable barrier is permeable to both sodium ion and to electrolyte. Such a permeable barrier is called a diaphragm. Diaphragms may be prepared from fibrous materials such as asbestos. They may, additionally, be treated to render them more durable, for example, by controlled heating, impregnation with various silicates, and treatment with various fluorocarbon compounds.
Alternatively, the barrier may be permeable only to cations and substantially impermeable to the bulk flow of electrolyte. Such a cation permeable barrier is referred to as the permionic membrane. Permionic membranes may be provided by various organic fibers and membranes, such as perfluoroalkyl resins having acid moieties. An effective permionic membrane is provided by an interpolymer of tetrafluoroethylene and trifluorovinyl sulfonic acid homologs, such as DuPont "XR" resin.
The catholyte liquor contains sodium ions, hydroxyl ions, and in the case of liquid permeable diaphragms, chloride ion.
In the monopolar electrolytic cells of the prior art, as well as in bipolar electrolytic cells of the prior art described, for example, in U.S. Pat. No. 3,337,443 to C. W. Raetzsch et al and in U.S. Pat. No. 3,563,878 to M. P. Grotheer, the anodes are graphite anodes.
In more recent chlor-alkali cell designs, the anodes have been provided by a valve metal, typically titanium, haing an electroconductive, electrocatalytic surface. Such titanium anodes, however, require significantly different cell bottoms and cell bottom geometry than is normally present in chlor-alkali cells intended for the use with graphite electrodes. For this reason, substantial redesign of the cell bottom and, in many instances, new cell bottoms are required to convert existing graphite anodeequipped diaphragm cells to titanium anodes.
A more recent development in chlor-alkali cell technology has been silicon anodes. Typically, the silicon useful in providing an anode for chlor-alkali service has an electrical conductivity greater than 100 (ohm-centimeters).sup..sup.-1 and substantial chemical resistance to acidic anolyte liquors under anodic conditions. The silicon alloys having the high electrical conductivity and the corrosion resistance required for electrochemical applications are characterized by the presence of a dopant and the presence of a transition metal. Typically, the dopant is present in an amount sufficient to provide an electrical conductivity in excess of 100 (ohm-centimeters).sup..sup.-1.
The silicon anodes useful in chlor-alkali electrolysis have a suitable electroconductive, electrocatalytic surface thereon.
In the mounting of silicon anodes in cell bottoms intended for use with graphite anodes, it has been found that cracking and fracturing occur in the silicon below the lead, at the silicon-lead interface and in the silicon above the upper surface of the lead. While these cracks and fractures may not immediately result in failure of the anode, they frequently provide a site for subsequent corrosion and failure.